1. Field of the Invention
The present invention relates to thin film composite membrane production via a transfer process and technology for use.
2. Description of the Art
For selective separation of fluids by permeation, membranes with very thin permselective active layers are desirable. As a consequence of Fick's first law of diffusion, flux (amount permeated per membrane area per time) is inversely proportional to active layer thickness. This relationship is represented generically in FIGS. 1A and 1B. As the thickness of the active layer is reduced, so is the membrane area that is required to obtain a specified quantity of permeate, and hence equipment costs are reduced. But very thin active layers are fragile, and need a mechanically strong porous support layer to preserve their integrity during fabrication and use. As described by R. W. Baker in Membrane Technology and Applications, 2nd ed., John Wiley & Sons, Ltd, 2004, the characteristics of the microporous support are very important. The pores of the support layer in contact with the active layer must be small enough to support the active layer under high pressure, smaller than the thickness of the active layer, but also must be close together so that the path length for permeate diffusion to the pores is short. When the thickness of the thin-film active layer is in the range of a few microns or less, the small size of the pores can contribute significant mass transport resistance. The effect of this additional mass transport resistance is to reduce flux and also separation factor below the values expected for the active layer alone.
Blume et al. U.S. Pat. No. 5,085,776 describe a method for designing composite membranes for gas separations having a microporous support coated with a permselective layer. The method involves calculating the minimum thickness of the permselective layer such that the selectivity of the composite membrane is close to the intrinsic selectivity of the permselective layer. The calculation indicates that the support resistance is preferably less than 10% that of the active layer. By this calculation, a support with a relatively high mass transport resistance should be paired with a relatively thick active layer to avoid significant reductions in the selectivity. The negative aspect being that thicker active layers result in significantly reduced flux.
Additionally, as described by M. Mulder, Basic Principles of Membrane Technology, 2nd ed., Kluwer Academic Publishers, 1998, most composite membranes are prepared by a process that applies a solvent solution of the polymer or pre-polymer by a coating technique to the support layer. The active layer is formed by evaporation of the solvent and cross-linking of the polymer. The problem with simply coating the receiving support layer directly with the active layer coating mixture is that the coating mixture can penetrate the pores of the support layer. After curing, the effective thickness of the active layer is the sum of the thickness of the film on top of the receiving support plus the depth of the penetration of the coating mixture into the pores of the receiving support. W. Gudernatsch et al., “Influence of composite membrane structure on pervaporation”, J. Membrane Sci., 61, p. 19-30 (1991) and I. Vankelecom et al., “Intrusion of PDMS top layers in porous supports”, J. Membrane Sci., 158, p. 289-297 (1999) describe the importance of the surface porosity characteristics of the support layer skin in contact with the active layer, the problem of pore penetration and various methods to attempt to avoid it. One technique is to reduce the pore size of the support to exclude the large polymer molecules of the active layer, but as described above, this use of very small pores restricts flow of the permeating vapors through the support layer and increases mass transport resistance. Another possibility is to use a casting solvent that does not wet the support material, and hence will not penetrate the pores, but this is a very restrictive requirement for membranes where permeation of non-aqueous species are desired.
The effects of using small pore size supports is evident in FIG. 2, which plots permeance against the inverse of the active layer thickness for polydimethylsiloxane (PDMS) thin-film composite (TFC) membranes described in the literature, for pervaporation of ethanol from dilute ethanol/water solutions. Since operating conditions (temperature, feed concentration, permeate pressure) varied, the reported flux data are recalculated as permeances (basically, the driving force-normalized flux) in order to have a common basis. Support layers vary, but it is clear that as the active layer thickness is reduced below about 5 microns, permeance falls well-below the trendline which extrapolates from the permeances of the thicker TFC membranes.
The use of a composite membrane comprising a thin, non porous active layer which can be used in conjunction with high pore size and high porosity supports to greatly reduce mass transfer resistance in the support and improve flux and separation factor of the composite is therefore desired. Additionally, removing the possibility of undesired interactions between the active layer's casting solvent and the material of the support layer would be beneficial in that these materials could be chosen independently, thus allowing a wider range of materials and solvents to be used.